If a three-phase electric motor is started directly or using star-delta starting, unpleasant side-effects such as mechanical impacts inside the machine or voltage dips in the power supply system may be encountered.
To avoid these adverse effects, a so-called soft starter may be used to start-up or ramp-down a three-phase electric motor. With a soft starter, mechanical loads in the operating mechanism can be reduced, causing also less strain to the power supply system. In this manner, simple and economically more efficient use of the motor and machinery and equipment connected thereto may be achieved.
A soft starter may need to be able to switch very large currents, especially if it has been adapted to start-up or ramp-down a larger motor. For example, SIEMENS soft starter SIRIUS 3RW44 is currently capable to operate a motor with a power of 710 kW at 400 V in an inline circuit, and up to 1200 kW at 400 V in an inside-delta circuit. This means that switching must be performed for single-phase currents exceeding 1000 A which is a very challenging task.
Contactors, especially air contactors, are commonly used in a soft starter to protect the soft starter against electric arc and to protect the motor and machinery and equipment connected thereto against adverse effects of a bypass.
The fact that contactors used in soft starters for the higher power range (for currents 29-1200 A and higher) are usually relatively large results in a correspondingly larger volume of the housing of a soft starter, thus increasing the material cost and making installation of the device more difficult and space-consuming.